Vibrating conduit sensors, such as Coriolis mass flowmeters and vibrating densitometers, typically operate by detecting motion of a vibrating conduit that contains a flowing material. Properties associated with the material in the conduit, such as mass flow, density, and the like, can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness, and damping characteristics of the conduit and the material contained therein.
It is well known to use vibrating meters to measure mass flow and other properties of materials flowing through a pipeline. For example, vibrating Coriolis flowmeters are disclosed in U.S. Pat. No. 4,491,025 issued to J. E. Smith, et al. of Jan. 1, 1985 and also Re. 31,450 to J. E. Smith of Nov. 29, 1983. These vibrating meters have one or more fluid tubes. Each fluid tube configuration in a Coriolis mass flowmeter has a set of natural vibration modes, which may be of a simple bending, torsional, radial, lateral, or coupled type. Each fluid tube is driven to oscillate at resonance in one of these natural modes. The vibration modes are generally affected by the combined mass, stiffness, and damping characteristics of the containing fluid tube and the material contained therein, thus mass, stiffness, and damping are typically determined during an initial calibration of the vibrating meter using well-known techniques.
Material flows into the flowmeter from a connected pipeline on the inlet side of the vibrating meter. The material is then directed through the fluid tube or fluid tubes and exits the flowmeter to a pipeline connected on the outlet side.
A driver, such as a voice-coil style driver, applies a force to the one or more fluid tubes. The force causes the one or more fluid tubes to oscillate. When there is no material flowing through the flowmeter, all points along a fluid tube oscillate with an identical phase. As a material begins to flow through the fluid tubes, Coriolis accelerations cause each point along the fluid tubes to have a different phase with respect to other points along the fluid tubes. The phase on the inlet side of the fluid tube lags the driver, while the phase on the outlet side leads the driver. Sensors are placed at two different points on the fluid tube to produce sinusoidal signals representative of the motion of the fluid tube at the two points. A phase difference of the two signals received from the sensors is calculated in units of time.
The phase difference between the two sensor signals is proportional to the mass flow rate of the material flowing through the fluid tube or fluid tubes. The mass flow rate of the material is determined by multiplying the phase difference by a flow calibration factor. The flow calibration factor is dependent upon material properties and cross sectional properties of the fluid tube. One of the major characteristics of the fluid tube that affects the flow calibration factor is the fluid tube's stiffness. Prior to installation of the flowmeter into a pipeline, the flow calibration factor is determined by a calibration process. During the calibration process, a known fluid is passed through the fluid tube at a given flow rate and the proportion between the phase difference and the flow rate is calculated. The fluid tube's stiffness and damping characteristics are also determined during the calibration process as is generally known in the art.
One advantage of a Coriolis flowmeter is that the accuracy of the measured mass flow rate is not affected by wear of moving components in the flowmeter, as there are no moving components in the vibrating fluid tube. The flow rate is determined by multiplying the phase difference between two points on the fluid tube and the flow calibration factor. The only input is the sinusoidal signals from the sensors indicating the oscillation of two points on the fluid tube. The phase difference is calculated from the sinusoidal signals. Since the flow calibration factor is proportional to the material and cross sectional properties of the fluid tube, the phase difference measurement and the flow calibration factor are not affected by wear of moving components in the flowmeter.
A typical Coriolis mass flowmeter includes one or more transducers (or pickoff sensors), which are typically employed in order to measure a vibrational response of the flow conduit or conduits, and are typically located at positions upstream and downstream of the driver. The pickoff sensors are connected to electronic instrumentation. The instrumentation receives signals from the two pickoffs and processes the signals in order to derive a mass flow rate measurement, among other things.
Typical Coriolis flowmeters measure flow and/or density through the use of a coil and magnet as a pickoff sensor to measure the motion of a meter's vibrating flow tube/tubes. The mass flow rate through the meter is determined from the phase difference between multiple pickoff signals located near the inlet and outlet of the meter's flow tubes. However, it is possible to measure flow using strain gages in place of coil/magnet pickoffs. For example, International Patent Application No. PCT/US2014/033188 entitled “Improved vibrating flowmeter and related methods” describes a number of embodiments wherein a vibratory flowmeter utilizes strain gages instead of coil/magnet pickoffs to calculate mass flow, and this application is herein incorporated by reference, in its entirety. A fundamental difference between the two sensor types is that coil/magnet pickoffs measure the velocity of the flow tubes and strain gages measure the strain of the flow tubes. One benefit to utilizing coil/magnet pickoffs is that no matter how flow is split between two flow tubes, the meter flow control factor (FCF) does not change. Therefore clogs or residue build-up in one of the flow tubes does not interfere with mass flow measurements. It is precisely this “benefit” of coil/magnet pickoffs that limits their use for detecting clogs or residue build-up in the flow tubes.
A problem, therefore, with prior art flowmeters is their inherent lack of obstruction or build-up detection. The embodiments described below overcome this and other problems and an advance in the art is achieved. The embodiments described below provide a flowmeter with strain gages configured to detect asymmetric flow between the flow tubes of a flowmeter for the purpose of detecting obstructions or residue accumulation in the flow tubes. By connecting various combinations of strain gages having varying placements and orientations on a flowmeter with various combinations of Wheatstone bridge circuits, flow asymmetry is rendered detectable and reportable.